1. Field of the Invention
The present invention relates to a wire bonding apparatus.
2. Prior Art
Semiconductor device assembling includes, as shown in FIG. 3, a process that connects, by a wire 5, a first bonding point 3, which is on a semiconductor chip 2 bonded to a lead frame 1, and a second bonding point 4, which is on the lead frame 1.
In this process, after the wire 5 passing through a capillary 6 is connected (bonded) to the second bonding point 4 in step (a), a clamper 7 is opened and this clamper 7 is raised together with the capillary 6 while the wire 5 is released, thus delivering the wire 5 from the capillary 6 in step (b). Next, in step (c), the clamper 7 is closed, and the clamper 7 is raised further together with the capillary 6 with the wire being held by the clamper 7, thus in step (d) cutting the wire 5 from the second bonding point 4. As a result, a fixed length of wire (called a xe2x80x9ctailxe2x80x9d) 5a is left in step (e) so as to extend from the tip end of the capillary 6.
In this process, since the shape and length of the tail 5a are the factors that affect the quality of a ball that is formed at the end of the tail 5a in the next wire bonding process, these parameters are extremely important in wire bonding.
The above-described wire bonding method is disclosed in, for instance, Japanese Patent Application Publication (Kokoku) No. H5-60657 (Laid-Open No. H630042135) and Japanese Patent Application Laid-Open (Kokai) No. H4-318943.
The above-described wire bonding method is performed by the wire bonding apparatus shown in FIG. 2.
In this bonding apparatus, a bonding head 11 is installed on an XY table 10 that is driven in X and Y directions on a horizontal plane. A raising-and-lowering mechanism 12 which is driven by a Z-axis motor (not shown) is installed on the bonding head 11 so that the raising-and-lowering mechanism 12 is moved up and down. A bonding arm 13, which has the capillary 6 at one end thereof, and a clamper supporting body 14 are fastened to the raising-and-lowering mechanism 12. The clamper 7 is mounted on the clamper supporting body 14 so that the clamper 7 is free to open and close.
Furthermore, a spool support (not shown) and a tension mechanism 21 are provided on the bonding head 11. The spool support holds a wire spool 20 around which the wire 5 is wound, and the tension mechanism 21 applies air tension to the wire 5. The wire 5 passes through the capillary 6 from the wire spool 20 via the tension mechanism 21 and clamper 7. The passage of this wire 5 is called the xe2x80x9cwire path.xe2x80x9d In FIG. 2, the reference numeral 22 refers to guide rails that guide a lead frame 1.
The wire bonding apparatuses described above is shown in, for instance, Japanese Patent Nos. 2617541 (Laid-Open No. H2-122639) and 2841126 (H4-320350).
Conventionally, various methods have been used to stabilize the shape and length of the tail 5a. In the above-described Japanese Patent No. 2841126, for such a purpose the speed at which the capillary 6 and clamper 7 are raised when the wire is cut is set at a constant low speed. However, with these conventional methods, there are limits to the stabilization of the shape and length of the tail 5a. As a result of experiments conducted by the inventors of the present application with reference to the reasons of this problem, it was ascertained that the distance L (see step (a) in FIG. 3) that is between the capillary 6 and clamper 7 had something to do with the shape and length of the wire tail.
As described above, the capillary 6 and clamper 7 are raised together; and when the clamper 7 is closed during this rising motion to cut the wire 5 from the second bonding point 4, the wire 5 undergoes plastic deformation and as a result is cut. In this case, a rebound force is generated in the wire 5, and this force causes deformation in the wire 5. This wire deformation tends to occur between the clamper 7 and the capillary 6, where there is such a degree of freedom that the wire 5 is not constrained.
In a conventional wire bonding apparatus, the distance L between the wire insertion hole side of the capillary 6 and the clamper 7 is approximately 3 mm to 5 mm. Accordingly, when the wire is cut, the wire 5 is twisted and bent between the capillary 6 and the clamper 7; and the wire 5 is deformed as shown by wave-shaped line in step (e) in FIG. 3. When the wire 5 is thus deformed, the shape and length of the tail 5a are unstable, and the quality of the ball that is formed next will be unstable or not consistent. Furthermore, as a result of the deformation in the wire such as twisting, bending, etc. between the capillary 6 and clamper 7, the shape of the wire loop becomes unstable, and this would cause short-circuiting of the bonded wires or cause the wires to fall over during molding.
The above-described rebound of the wire during wire cutting will be described with reference to FIG. 4.
Step (a) in FIG. 4 shows the same state as step (c) in FIG. 3. In other words, in step (a), the capillary 6 and clamper 7 have risen together, and the clamper 7 has closed at an intermediate point during this rising movement so that the wire 5 is held by the clamper 7. When the capillary 6 and clamper 7 are raised further in step (b) in FIG. 4 from the position in step (a), the portion of the wire 5 that is between the bonded root 5b of the second bonding point 4 and the clamper 7 elongates, and the bonded root 5b undergoes plastic deformation. As a result, the wire 5 is cut. When the wire 5 is cut from the bonded root 5b, a rebound force is generated in the wire 5, and the elongated wire 5 extending from the clamper 7 to the lower end of the cut wire 5 tends to return to its original state as shown in step (c) or (d). Then, the wire is eventually deformed into an S shape or V shape in step (e) between the capillary 6 and clamper 7.
Accordingly, the object of the present invention is to provide a wire bonding apparatus that prevents deformation in the wire during wire cutting, so that the shape and length of the wire tail are stabilized, and the constant shape of the wire loop is obtained.
The above object is accomplished by a unique structure for a wire bonding apparatus in which a wire is passed through a capillary provided at one end of a bonding arm, and a clamper that clamps the wire is disposed directly above the capillary; and the unique structure of the present invention is that the distance between the upper surface of the capillary and the undersurface of the clamper is set so as to be very short.
More specifically, the distance between the upper surface of the capillary and the undersurface of the clamper is set to be 1.5 mm or less in the present invention.